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Definitions of Ion Sources
A day out at the zoo…
Richard Scrivens
CERN
CAS on Ion Sources, May 2012
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An Approximate Classification System
Try to break down ion sources into a few groups:
• Electron bombardment
• Plasma Discharge
• RF discharge
• Microwave and ECR
• Laser Ion Sources
• Surface
• High Charge State Sources
• Charge Exchange Ion Sources
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Electron Bombardment Ion Source
•
•
•
•
Generate electrons with a cathode.
Accelerate them with a cathode – anode potential difference.
The ions impact on neutral atoms and molecules to ionize them.
Electrons can be confined (path length increased) by magnetic fields.
Beam extraction
Anode
Cathode
Ionization Region
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Electron Bombardment Ion Source
Energy
Electrons within a material are heated to energies above that needed to escape the material.
Energy difference between
highest energy electron and vacuum
Work Function φs
Electrons
Material
Cathode emission is dominated by the Richardson Dushmann equation.
Cs
100
⎛ − eφ s ⎞
J = A ⋅ T exp ⎜
⎟
⎝ kT ⎠
LaB6
Cs/O/W
Mixed Oxide
Thoriated W
2
Ta
10
-2
Emission (Acm )
W
4π em e k
6
-2
−2
A=
≈
1
.
2
×
10
Am
K
3
h
2
1
W
Thoriated W
Mixed Oxide
Caesium
Ta
Cs/O/W
LaB6
0.1
0.01
In practice, A is a (temperature independent) value, that is material dependent. 1E-3
500
1000
1500
2000
2500
Temperature (K)
5
Electron Bombardment Ion Source
Ionization Cross Section by Electron Impact
10
H
He
Ne
Ar
Kr
Xe
1
Lcollision
10
Cross Section (cm2)
σ .n atom =
-15
10
10
10
-16
-17
R. Rejoub, B. G. Lindsay, and R. F. Stebbings, Phys. Rev. A 65, 042713 (2002)
Except H: Y.‐K. Kim and M.E. Rudd, Phys. Rev. A 50, 3954 (1994)
-18
-19
10
1
Some cross section data available in:
http://physics.nist.gov/PhysRefData/Ionization/Xsection.html
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10
Electron Energy (eV)
10
3
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Electron Bombardment Ion Source
• The FEBIAD ion source uses a grid near the cathode to provide a electron acceleration followed by a lower E field drift.
• This reduces the electron energy distribution and the ion distribution.
• It also reduces the electric field pulling the ions back towards the cathodes.
• Internal source pressures are usually low (10‐5 to 10‐2 mbar).
• Intensity from Electron Bombardment is limited before a strong plasma is formed (where the source becomes a plasmatron).
R.L. Gill, A. Piotrowski, NIM A, Vol 234 (2), p213, 1985.
http://dx.doi.org/10.1016/0168‐9002(85)90907‐6
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Plasma Discharge Sources
• Van Ardenne pushed the operating regime of the electron bombardment ion sources towards the plasmatron.
• Driven either by a cold cathode, or forced by a hot cathode and thermionic emission.
• A sustained discharge is possible once above the Paschen line for a gas.
V cathode‐anode
105
Breakdown
Mean free path for electrons too short to gain enough energy to cause ionization
Too little gas density
3
Probability of 2
10
10
electron+atom
ionization too low
102
10‐1
100
102
Pressure x distance (Torr cm)
103
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Plasma Discharge Sources
• Von Ardenne pushed the operating regime of the electron bombardment ion sources towards the plasmatron.
• In Plasmatrons and Discharge sources the density of ions (and electrons) is high enough such that a plasma is formed.
1
10
0
10
-1
10
Debye Length (m)
Electric fields can permeate a plasma up to the size of the Debye length.
1eV
3eV
10eV
30eV
100eV
-2
10
-3
10
-4
10
ε 0 kTe
λD =
-5
10
ne e
2
-6
10
8
10
10
10
12
10
14
10
16
10
18
10
20
10
Electron Density (m-3)
• Applied Electric and magnetic fields shape the plasma, but they are also affected by it.
• Electron Bombardment is still the principle route for ionization.
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Plasma Discharge Sources
Multicusp magnets
e- (hot)
Filter magnets
H2(ν>0)
H2(ν=0)
e- (hot)
H2(ν>0)
ions
efilament
H-
e- (cold)
Sketch: M Stockli,
R Welton, SNS
26 cm
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Plasma Discharge Sources
A rich sub‐species…
The cathode‐anode‐B field configuration can be varied to produce different source types:
C: Cathode
• Multicusp discharge sources.
E
A
A: Anode
• Plasmatron
E: Extraction Electrode
C
• Duoplasmatron
B: Magnetic field
• Magnetron
: Plasma
E
: Beam
A
• Penning
: Magnetic steel
B
C
C
C
B~
E
E
A
B
A
C
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Cathodes – problem 1 – Ion Sputtering
The ions formed in the plasma are attracted to the cathode, and sputter material from it. Heavier atoms sputter much more. Tungsten is one of the hardest materials.
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Cathodes – problem 2 – Surface changes
• Gases and other materials in the source can cover the cathode.
• This changes the surface, affecting its emission properties (usually for the worse) .
• Mixed material cathodes – elements sputter and evaporate at different rates.
Cs
100
LaB6
Cs/O/W
Mixed Oxide
Thoriated W
Ta
-2
Emission (Acm )
10
W
1
W
Thoriated W
Mixed Oxide
Caesium
Ta
Cs/O/W
LaB6
0.1
0.01
1E-3
500
1000
1500
2000
2500
Temperature (K)
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RF Discharge
• Instead of using a cathode ‐> anode potential to create an electric field, the electric field from an RF system can be used.
• Electrons are accelerated by the electric field. Usually there is no wave‐plasma resonance…
• Two possibilities
E
• Solenoid antenna – a circular electric field is generated around a solenoidal magnetic field (also could be saddle like)
E≈
μ 0π N t Irf
L
B
r: solenoid radius
N: solenoid turns
L: solenoid length
I: Peak current
• Capacitive coupling – the electric field runs between 2 conductors –
magnetic field is elsewhere
E
rf
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RF Discharge
• Even the solenoid antenna based systems can produce electric fields of several kV/m.
• As these electric fields do not terminate on a surface, not enhanced by a cathode tip, there is less surface sputtering.
• As there is no resonant heating of electrons, they do not reach too high energies (useful for producing low charge states, or negative ion sources).
SNS External antenna source
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ECR ion sources
In an Electron Cyclotron Resonance Ion Source (ECR or ECRIS):
• There is a magnetic field, and the injection of RF or microwaves.
• Wave frequency satisfies the ECR resonance somewhere in the source (f=28 GHz / Tesla)
• The electrons absorb energy from the Right Hand Circular Polarised wave
(EM waves can always be decomposed into RHCP and LHCP)
• The electron cycltron radius is small compared to the source volume.
• The ECR ion source has no cathode (overcoming the cathode problems).
• Solenoidal fields are used, in order to place a magnetic field over the whole source chamber.
• Magnets can be permanent, to reduce size and power consumption.
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ECR ion sources
eB
fc =
2πγ m e
fc
= 28 GHz/Tesla
B
ρc =
βγ m e c
eB
-2
10
30
gamma=1
gamma=1.1
gamma=1.2
20
-3
10
Radius (m)
frequency [GHz]
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1eV
10eV
100eV
1000eV
15
-4
10
10
-5
10
5
0
-6
0
0.2
0.4
0.6
B field [Tesla]
0.8
1
10
0
0.2
0.4
0.6
0.8
1
B (T)
Note: Only energy perpendicular to B field
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ECR ion sources
Solenoids (B field)
Plasma Volume
Extraction System
RF injection
High intensity H+ D+ source, up to CW operation. CEA – SILHI 2.45GHz ECR source for H+, D+
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Laser ion sources
The interaction of light with matter can give rise to two types of ion sources:
Laser Ionization Ion Sources
Laser Plasma Ion Sources
These two techniques are very different in the way that they produce ions.
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Laser Ionization ion sources
• It is not easy to ionize atoms directly with a laser.
hc
1 .24
=
λ=
μm
eΦ i
Φi
• For the lowest ionization potential (Francium, 3.83eV) this is already UV light.
• Usually necessary to use at least two steps (or more), first to an excited state (requiring a tunable laser) and then to ionize.
• This allows the excitation to be chemically selective.
• Typically the source works continuously with low ion currents (needing a CW laser).
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First Ionization Potential (eV) of all elements
3.83
3.894
4.177
4.341
5.139
5.17
5.212
5.279
5.392
5.4259
5.46
5.53
5.54
5.554
5.58
5.64
5.67
5.695
5.786
5.86
5.89
5.94
5.986
5.993
5.999
6.018
6.02
6.05
6.06
6.08
6.101
6.108
6.113
6.15
Francium
Cesium
Rubidium
Potassium
Sodium
Actinium
Barium
Radium
Lithium
Lutetium
Praseodymium
Neodymium
Cerium
Promethium
Lanthanum
Samarium
Europium
Strontium
Indium
Terbium
Protactinium
Dysprosium
Aluminum
Americium
Gallium
Holmium
Curium
Uranium
Plutonium
Thorium
Erbium
Thallium
Calcium
Gadolinium
6.184
6.19
6.23
6.254
6.3
6.38
6.42
6.5
6.54
6.58
6.65
6.65
6.74
6.766
6.82
6.84
6.88
7.099
7.28
7.289
7.344
7.37
7.416
7.435
7.46
7.576
7.635
7.646
7.726
7.86
7.87
7.88
7.89
7.899
Thulium
Neptunium
Berkelium
Ytterbium
Californium
Yttrium
Einsteinium
Fermium
Scandium
Mendelevium
Hafnium
Nobelium
Vanadium
Chromium
Titanium
Zirconium
Niobium
Molybdenum
Technetium
Bismuth
Tin
Ruthenium
Lead
Manganese
Rhodium
Silver
Nickel
Magnesium
Copper
Cobalt
Iron
Rhenium
Tantalum
Germanium
7.98
8.151
8.298
8.34
8.42
8.641
8.7
8.993
9
9.009
9.1
9.225
9.322
9.394
9.65
9.752
9.81
10.36
10.437
10.451
10.486
10.748
11.26
11.814
12.13
12.967
13.598
13.618
13.999
14.534
15.759
17.422
21.564
24.587
Tungsten
Silicon
Boron
Palladium
Polonium
Antimony
Osmium
Cadmium
Platinum
Tellurium
Iridium
Gold
Beryllium
Zinc
Astatine
Selenium
Arsenic
Sulfur
Mercury
Iodine
Phosphorus
Radon
Carbon
Bromine
Xenon
Chlorine
Hydrogen
Oxygen
Krypton
Nitrogen
Argon
Fluorine
Neon
Helium
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Laser Plasma ion sources
• A pulsed laser beam is focused onto a target.
• At some position the laser frequency couples to the free electron plasma frequency (either in the material, or the formed plasma).
• In the dense plasma, ions of the target material are formed through electron impact ionization.
2πc
λ=
e
m eε 0
ne
• ne=1021 cm‐3 corresponds to 1µm.
• Laser power density needs to be above 106 W/cm2, which is easily available with pulsed lasers.
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Laser Plasma ion sources
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lasers (3x10
2
W/cm ) focused onto the rear of a thin • The future may be PetaWatt
(CH) target, have produced 58MeV protons – source and accelerator in one!
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Surface Ion Sources
• Inside a solid state material, conduction electrodes are confined by the charge of the ions of the material.
• They are confined within the material by a binding energy (called the Fermi Energy).
• Energy in excess of the Fermi Energy must be applied in order to liberate an electron (e.g. through the photo‐electric effect, or by heating). Energy difference between
highest energy electron and vacuum
Work Function
Energy
Electrons
Material
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Surface Ion Sources
• Similarly the electron is trapped on the atom.
• But if a low ionization potential atom is in contact with a high work‐function material, the electrons energetic preference is to be in the material.
Material
e.g. Rb
Φi=4.2V
Material
• The material needs to be hot enough to evaporate the ions.
e.g. W
• The Saha‐Langmuir equation predicts the ratio of ions to neutrals from the surface.
Φs=4.54V
⎛
ni
g 0 (φi −φ s ) kT
= ⎜⎜ 1 +
e
n 0 + ni ⎝
gi
Energy
⎞
⎟⎟
⎠
−1
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Surface Ion Sources
⎛
ni
g 0 (φi −φ s ) kT
e
= ⎜⎜ 1 +
n 0 + ni ⎝
gi
0
Cs
⎞
⎟⎟
⎠
−1
Rb
Example ion fraction for Φs=4.54eV
Points are marked at 1000 C
Ba
W
Li
10
T=0 C
T=1000 C
-1
10
-2
10
-3
ni/no+ni
10
-4
10
-5
10
-6
10
-7
10
-8
10
3.8
4
4.2
4.4
4.6
4.8
First Ionization Potential (eV)
5
5.2
5.4
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Surface Ion Sources
Extractor / puller electrode
Gas / material inlet
Heated high work function tube (e.g. W)
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Surface Ion Sources – negative ions
Gas/Atoms
e.g. H
Φa=0.75V
Material
• If the base material has a low work‐function, and the atoms a high electron affinity‐
e.g. LaB6
• The electrons have a finite probability of being present on the electron.
Φs=2.6V
• Use plasma bombardment to liberate the atoms from the surface. Energy
• Use an alkali metal coating deposited on the surfaces inside a plasma discharge source, to produce the ions.
• The Saha‐Langmuir equation can also be used for this case.
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Negative ions – Why?
• Negative ions are (generally) much harder to produce than positive ions.
• Their benefits for the following accelerator are:
• They have the opposite charge (so are oppositely affected by E and V fields).
• They are easily stripped to positive ions or neutrals (normally at higher energies).
HV terminal
Tandem
• Negative accelerated to foil, positive ion back to ground.
E
E
H‐ ‐> p
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Negative ions – Why?
Extraction (from cyclotrons)
• Change the charge in a foil, and the positive ion extracts itself H‐
p
H‐ ‐> p
Charge exchange injection (to synchrotron).
p
p
• Overlap the negative and (circulating ) positive ions – strip to positive – overcome Louiville!
H‐
H‐
foil
Gas Neutraliser
H0
B field
High Energy Neutral Beams (Magnetic Confinement Fusion)
• Efficient stripping to neutrals – to inject through a magnetic field.
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Highly Charged Ions
• For heavy ions it is often necessary to create high charge states.
• The high charge state allows more energy to be gained in the accelerating electric field.
• To create highly charged ions need:
• Direct ionization with a single electron impact is not feasible
(A ‐> An+).
• So multi‐step ionization is the only practical solution (An+ ‐> An+1+).
• High Energy electrons – above the ionization potential of the ion charge start required.
• Ions to remain in the plasma long enough for sufficient ionizing collisions to take place (long time, or dense plasma).
• 3 types of sources are generally used, we have already seen them:
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Highly Charged Ions ‐ ECRs
• The electrons are heated using the Electron Cyclotron Resonance.
• The Magnetic field I formed with co‐linear solenoids, with a higher magnetic field at the ends, the confine the electrons with a magnetic mirror.
• Radially the confinement is made with a multi‐cusp magnet.
• The inverse dynamics of collisional drift means the hot electrons are well confined.
• This leads to a charge density, that traps ions, sufficiently long for mulit‐step ionisation to take place.
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B
axial field
Highly Charged Ions ‐ EBIS
Axial magnetic field
EBIS cross‐view
• The electrons are accelerated from a cathode into a drift region (usually in a solenoid field to increase the current density).
Axial potential
• Ions are trapped radially by the electron beam potential.
• Longitudinal trapping is done with electrodes.
Radial potential
• Neutral gas or 1+ ions are injected, and confined long enough for multi‐step ionzation.
110
Sn
27+
140
Exracted current (pA)
• When the ion charge state is reached, one of the electrode barriers is reduced to allow the ions to escape.
160
26+
28+
120
25+
100
80
29+
60
24+
A/q=4
40
14
20
N
3+
23+
0
77
79
81
83
85
87
B-field (mT )
89
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Highly Charged Ions – Laser Ion Sources
• Use the Laser Plasma Ion Source with a high power laser.
• Generate a very dense, hot plasma by coupling to the plasma frequency.
• Ions travel through the plasma, and due to the high density, they still undergo a large number of collisions.
• Spectrum from the Laser Ion Source :
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Charge Exchange Ion Sources – Negative Ions
• Ions travelling through a gas can undergo a double charge exchange, to produce negative ions.
• The cross section for the charge exchange is very high for good ‐ electron doner ‐
alkali metals, • Although there are many sources for H‐, not so many are well suited to other negative ion types.
Schlachter and Morgan, AIP Conf. Proc. 111, p149 (1984)
Schlachter, AIP Conf. Proc. 111, p300 (1984)
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Charge Exchange Ion Sources – Negative Ions
High intensity ion source
Metal Vapour Ionizer (Li, Na, Cs)
He+
• In this case He‐ ions are created in a metal vapour cell, to be later re‐
neutralized and injected as a probe beam into ITER.
• If the outer electrons on the metal vapour are polarised, this can be transferred to the ion, which can be made to transfer the spin to the nucleus.
He‐
To second acceleration stage and neutraliser
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Ion Sources – what can we conclude?
• There are a lot of species of ion sources.
• Inside each species, there are sub‐species and different implementations.
• Just in this overview, there have been a lot of physical processes shown (if not yet understood), in the next days you will learn even more.
• We are now entering the Ion Source Enlightenment – we can now start to model the multiple physics processes in our sources on a desktop computer, and understand how our sources are really working…
• Get involved, and become the new Spinoza, Newton, Voltaire…
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Symbols Used
A: Richardson Dushmann constant.
B: Magnetic Field
c: Speed of light.
e: Electron charge
h: Plank Constant
f: Frequency
fc: Cyclotron Frequency
β: Relativistic Beta
εo: Free space Permittivity
Φs : Work Function
Φi : Ionization Potential
γ: Relativistic Gamma
λ: Wavelength
νc : Collisional frequency
ρc: Cyclotron radius
J: Current density
k: Boltzmann constant
me: Electron mass
ne: Electron density
ni: Ion Density
n0: Neutral Atom Density
Nt: Number of turns
T: Temperature
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